Devices are already known which permit the individual measurement of velocities, velocity fluctuations, or diameters of submicronic particles. However no apparatus known at present permits the simultaneous determination of the three above-mentioned parameters for a given fluid.
An example of a prior art device which uses laser techniques for measurements of sundry dynamic properties of fluids is disclosed in the article by John C. Angus et al. "Motion Measurement by Laser Doppler Techniques", in Industrial and Engineering Chemistry, Vol. 61, No. 2, Feb. 1969, p. 8 et seq.
The proposed apparatus disclosed by Angus et al is designed to detect a given spectrum, but its design makes it unsuitable for the simultaneous recording of a homodyne spectrum and a heterodyne spectrum.
In order to arrive at an experimental determination of the three required quantities (velocities, velocity fluctuations and particle diameters), it is necessary at present to use two devices, the first being used to measure velocities and velocity fluctuations of the fluid, the second used to determine the diameters of the particles contained in the said fluid.
The fact that according to the previous technique it is necessary to use two devices has its disadvantages. On the one hand it means that two measurements have to be made which are separated by a considerable time interval, i.e., one velocity and velocity fluctuation is determined at a time t and a particle diameter is determined at a time t.sub.1. But between these two measurement times the conditions of the experiment may have varied and there is never any certainty whether the measured velocity corresponds to the diameter determined at a later time. Furthermore, the fact that two measurements have to be made necessitates an analysis time which is much too long, and may be as much as one day. The above-mentioned disadvantages can be avoided by means of the invention. In fact it permits the simultaneous measurement of velocities, velocity fluctuations and particles diameters of the fluid in question. Consequently the three required quantities can be determined at the same time t.
Furthermore, the invention permits a reduction in operation time, allowing a spectrum to be obtained within a period of a few minutes (and even a few seconds). The spectrum which is thus obtained can then be used to obtain the actual measurement of the three factors required.
The invention relates to a method and apparatus for the simultaneous measurement of velocities, velocity fluctuations, diameters and other data concerning submicronic particles in a state of flow in a fluid wherein two parallel beams of coherent light are produced from a single laser beam, the beams are focused in the fluid zone to be measured, and the light issuing from the zone is spectrally analysed. According to the present invention, a homodyne spectrum is first recorded by a frequency sweep over a first frequency range of the diffused light received from one of the parallel beams at a first detection angle, and a heterodyne spectrum is then recorded by continuing the frequency sweep over a second frequency range of the diffused light received from the one beam and the light received from the other, previously blocked beam at a second detection angle. A frequency f.sub.c is defined, termed the critical frequency, higher than the highest frequency of the homodyne spectrum, which is used as a control parameter to control the emission and reception of the coherent light beams.
The data of the two spectra is then processed in order to simultaneously determine the values for velocities, velocity fluctuations and particle diameters within the zone in question.
According to the present invention, two beams from the same laser source are focused in a spatial zone encompassing the fluid to be analysed.
A spectrum analyser sweeps the signal produced by photodetector apparatus by pass-bands (frequency sweep) within periods exceeding 0.1 ms by several orders of magnitude. According to an essential characteristic of the invention, the analyser first works on one beam and then, from the critical frequency, on the two beams.
The total spectrum recorded contains all the required data: the homodyne spectrum permitting determination of particle diameter, and the heterodyne spectrum permitting determination of the velocities and velocity fluctuations of the particles in the fluid.
These and other features and advantages of the present invention are disclosed in or apparent from the following detailed description of preferred embodiments.